TPS64200,TPS64201

TPS64202,TPS64203

SLVS485 − AUGUST 2003

www.ti.com

14

Selecting the External PMOS

An external PMOS must be used for a step-down converter with the TPS64200. The selection criteria for the

a low voltage. As the gate of the PMOS finds the full supply voltage applied to the TPS64200, the PMOS must

be able to handle that voltage at the gate. The drain to source breakdown voltage rating should be at least a

few volts higher than the supply voltage in the application. The rms-current in the PMOS assuming low inductor

current ripple and continuous conduction mode, is:

I

PMOS(rms) [

I

O

D

+ I

O

V

O

V

I

(11)

The power dissipated in the PMOS is comprised of conduction losses and switching losses. The conduction

using:

P

(cond) +

I

O

D

2

r

DS(on)

1

) TC

T

J

–25

°C [ I

O

D

2

r

DS(on)

with TC = 0.005/

°C

(12)

Table 2. PMOS Transistors Used in the Application Section

TYPE

MANUFACTURER

rDS(on)

VDS

ID

PACKAGE

Si5447DC

Vishay Siliconix

0.11

Ω at VGS = −2.5 V

−20 V

−3.5 A at 25

°C

1206

Si5475DC

Vishay Siliconix

0.041

Ω at VGS = −2.5 V

−12 V

−6.6 A at 25

°C

1206

Si2301ADS

Vishay Siliconix

0.19

Ω at VGS = −2.5 V

−20 V

−1.4 A at 25

°C

SOT23

Si2323DS

Vishay Siliconix

0.41

Ω at VGS = −2.5 V

−20 V

−4.1 A at 25

°C

SOT23

FDG326P

Fairchild

0.17

Ω at VGS = −2.5 V

−20 V

−1.5 A

SC70

Selecting the Output Diode

The output diode conducts in the off phase of the PMOS and carries the full output current. The high switching

frequency demands a high-speed rectifier. Schottky diodes are recommended for best performance. Make sure

average current in the diode in continuous conduction mode, assuming low inductor current ripple, is:

I

(diode)(Avg) [

I

O

(1–D)

+ I

O

1–

V

O

V

I

(13)

Table 3. Tested Diodes

TYPE

MANUFACTURER

VR

IF

PACKAGE

MBRM120LT3

On Semiconductor

20 V

1 A

DO216AA

MBR0530T1

On Semiconductor

30 V

0.5 A

SOD123

ZHCS2000TA

Zetex

40 V

2 A

SOT23−6

B320

Diodes Inc.

20 V

3 A

SMA